When using rectangular film formats like the 35 mm format, images are recorded on film with a “landscape” (horizontal) orientation in respect to the common way of holding a camera. When the photographer wishes to capture a portrait he will tilt the camera 90 degrees and thus acquire an image with a “portrait” (vertical) orientation. Later when the developed images are viewed, the viewing person will manually orient them correctly. Since the images are on paper, it is relatively easy to reorient some of them.
In digital photography the landscape orientation is the default setting for most cameras. When the captured images are viewed on a display, they will appear with a landscape orientation with no respect to whether the images were actually captured with the camera held in a portrait or landscape orientation. The images then have to be manually inspected and later possibly rotated to reflect their original orientation. Some digital camera manufacturers are now beginning to include a sensor unit, which detects whether the camera is placed in landscape or portrait position when an image is captured.
In U.S. Pat. No. 5,900,909 an orientation detector which consists of two mercury tilt switches is described. The two mercury switches make it possible to determine whether the user is holding the camera in the normal landscape orientation or in a portrait orientation. There are two portrait orientations: One is the result of a clockwise rotation whereas the other is the result of a counter clockwise rotation. The use of mercury switches has some distinct disadvantages in that mercury can cause great damage when it interacts with the human body, and for that reason it is quite unpopular in many products. Mercury switches usually consume a lot of space in comparison with monolithic IC's. This is due to their very mechanic structure, which makes miniaturisation difficult. In a digital camera it is crucial to minimise the size and weight, so in respect to this, the use of mercury and other primarily mechanically based switches, is not the optimum choice. A mercury switch based solution in a digital camera is limited to detecting a few rough orientations, i.e. landscape and portrait. The robustness and ease of use of the mercury switch are its primary advantages today.
The main limitation regarding micro-mechanical accelerometers fabricated in e.g. silicon is related to their ability to absorb shock without being damaged.
Taking pictures with long shutter times and maybe even a high degree of zoom makes the image capture process very sensitive to vibrations, which will result in blurred images. At short shutter times the image is less likely to be affected by vibrations since most vibrations, which will affect a camera, have an upper frequency limit, due to mechanical damping from the surroundings. Especially handheld photography easily results in blurred images when longer shutter speeds are used. One solution to the described limitations is to be able to compensate for most vibrations. Vibrations can be compensated optically by means of a lens module, which is capable of moving the projected image around in the image focus plane. This requires a special and expensive lens.
When vibrations cannot be compensated, another way of helping the photographer to acquire the optimum images is to inform him about any possibility of blurring, which may have occurred in a captured image. With feedback from the camera regarding the degree of shaking during the exposure time, it is possible for the photographer to decide whether he wants to capture another image of the same scene.
In U.S. Pat. No. 4,448,510, a camera shake detection apparatus is described. It includes an accelerometer, which is connected to a control circuit, which activates an alarm, when the acceleration exceeds a certain predefined threshold level. The threshold level can be influenced by the exposure time—a long exposure time results in a low threshold level and vice versa for a short exposure time. The output from the accelerometer may also be forced through an integrator before comparing the output to a threshold level to account for the fact that blurring is more probable to occur if a large number of high accelerations are detected. None of the described implementations are able to determine if the camera after a short period of vibrations returns to its initial position or the position where the majority of the exposure time has been spent. In such a case the suggested implementations would generate a “blur” alarm, even though the image could be sharp.
In some applications, especially the more technically oriented, it can be an advantage to have knowledge about how the camera is physically oriented in space. In a set-up with a digital camera connected to a GPS receiver, knowledge about the roll and pitch of a camera can be used to automatically pin point the scene being photographed. This can be used in aerial photography and other related technical applications. In other set-ups, feedback to the photographer about the exact roll and pitch can be useful for him to correct his orientation of the camera. Another use of the roll information is to automatically correct for small degrees of slant in the sideways direction. In most common photographic situations it is not desirable to have an automatic correction of a slight slant, as the photographer often wants full control of the image orientation. A feature like automatic slant correction should be user configurable in the sense that it can be turned off and on.
JP 58-222382 discloses an apparatus that automatically corrects inclination of scanned originals by changing the address where the image data is written to reflect the original with no inclination. Inclination is measured by using feedback from a couple of timing marks, which are connected to the slant of the original. Measuring the inclination through the use of timing marks is not useful in digital still photography. General image rotation in software is carried out by moving the original image data to a new position in another image file/buffer.
The present invention may be implemented in a digital still camera or a digital still camera back and supply a total solution which is very compact, consumes little power, and is applicable in a variety of digital still camera applications. The use of a single detector unit for a variety/plurality of functions decreases the physical size, lowers the power consumption, and keeps the price down. The use of a micro-mechanical accelerometer as opposed to a mercury switch has the distinct advantage that it does not contain mercury.
The micro-mechanical accelerometer has several advantages over the mercury switch and the pendulum based orientation detector. Some of these advantages are:
it can easily be miniaturised,
it is a measurement device with a high degree of accuracy which can be configured dynamically for a variety of applications through the use of different processing which can be integrated in a digital processing unit or analogue electronics,
it may be applied to measure both static and dynamic acceleration at the same time. In comparison, the mercury switch and the pendulum are both optimised for measuring static orientation.
With the integration of more than one measurement axis in a silicon-based chip it becomes possible to measure both dynamic and static acceleration in several directions at the same time. The static acceleration is basically obtained by low-pass filtering the raw outputs from the accelerometer(s). More sophisticated filtering can be applied to handle specific requirements. With static acceleration from at least two axe—which are perpendicular to each other—it is possible to obtain the precise degree of both roll and pitch for a digital still camera. This may be used in technical applications for automatic or manual correction of slant in both sideways and forwards directions. Mercury switches or pendulums are limited to a more rough evaluation of the orientation of the camera (basically limited to two positions).
A subset of the before-mentioned static acceleration measurement feature is the possibility to automatically determine when an image should be displayed with portrait or landscape orientation. The high precision of the roll and pitch information makes it possible to determine the correct orientation under the most difficult conditions where a slight mechanical tolerance for a mercury switch or pendulum based solution easily would result in an unexpected determination of orientation.
The mercury switch and pendulum switch based solutions lack the possibility to be dynamically configured to each users need, as their functionality is fixed mechanically when they leave the factory. An example of this could be a user who wishes that his camera should display images with a landscape orientation until he tilts the camera 75 degrees, whereas the normal configuration would be to display an image with a portrait orientation when the camera is tilted more than 45 degrees.
The measurements of dynamic acceleration (vibration) during the time of exposure may be used in a variety of ways to reduce the possibility of the photographer taking a blurred image. The use of active compensation for camera movements can be used to extend the previous working range for photography in terms of longer exposure time, more zoom, and the ability to capture images in vibration dominated surroundings, i.e. helicopters.
With a traditional film camera it is necessary to have an expensive lens which corrects the induced vibrations by changing the optical path of incident light. When the vibrations are compensated either by plain image processing with input from the recorded movements, or by active compensation through movement of charges in the image sensor, or by physically moving the image sensor itself, all the outlined compensation solutions described in detail below, enable the use of any type of lens, and are still able to reduce blur. The addition of a little extra image processing to compensate for vibrations through post-processing, or the use of charge movement in the sensor, does not increase the manufacturing cost, as opposed to a solution which changes the optical path.
When using accelerometers, generation of a “blur” warning is much more fail safe than earlier solutions which were not able to determine if the camera after a short period of vibrations would return to its initial position or the position where the majority of the exposure time had been spent. In such a case the earlier implementations would generate a “blur” alarm, even though the image could be sharp.